Method of manufacture of an exhaust gas sensor for an air-fuel ratio sensing system

ABSTRACT

A titanium dioxide ceramic disc having two spaced electrodes embedded therein is located in the exhaust passage of a reciprocating internal combustion engine. The electrical resistance across the electrodes is proportional to the equilibrium oxygen pressure of the exhaust gases and thus represents the air-fuel ratio of the mixture supplied to the engine.

United States Patent 1191 Stadler et al.

[ METHOD OF MANUFACTURE OF AN EXHAUST GAS SENSOR FOR AN AIR-FUEL RATIOSENSING SYSTEM [75] Inventors: Henry L. Stadler; Tseng-Ying Tien,

both of Ann Arbor; Michael J. Esper, Detroit; Donald J. Romine,Southfield, all of Mich.

[731 Assignee: Ford Motor Company, Dearborn,

Mich.

[221 Filed: Aug. 23, 1973 [21] Appl. No.: 391,424

Related US. Application Data [63] Continuation of Ser. No, 198,515, Nov15, 1971,

abandoned.

[52] US. Cl. 29/592; 23/232 E, 23/254 E; 29/625; 156/89 {51] Int. Cl.GOln 29/02 [58] Field of Search 23/232 E, 254 E, 255 E;

204/195 P, 195 M, 195 S; 73/27 R, 26; 338/34, 249, 254, 255, 256, 314;29/592, 610, 612, 613, 614, 625, 627; 156/89, 228,

[56] References Cited UNITED STATES PATENTS 2,389,420 11/1945 Deyrup29/625 UX 1 July 8,1975

2,427,212 3/1948 Schottland 29/625 UX 2,470,509 5/1949 Marini A 29/625UX 3.040113 6/1962 Byer et a1, 29/625 UX 3,189,974 (3/1965 Fabricus29/625 UX 3,189,978 6/1965 Stetson 29/625 3,518,756 7/1970 Bennett etal. 29/625 3,604,082 9/1971 McBrayer et al. 65/59 UX 3,695,848 10/1972Taguchi 1 23/254 E 3,751,968 8/1973 LOh el al .1 23/254 E X PrimaryExaminer-C. W. Lanham Assistant Examinerloseph A. Walkowski Attorney,Agent, or Firm-Robert A. Benziger; Keith L. Zerschling [57] ABSTRACT Atitanium dioxide ceramic disc having two spaced electrodes embeddedtherein is located in the exhaust passage of a reciprocating internalcombustion engine. The electrical resistance across the electrodes isproportional to the equilibrium oxygen pressure of the exhaust gases andthus represents the air-fuel ratio of the mixture supplied to theengine.

6 Claims, 3 Drawing Figures P'A'TENTEBJuL e ms 3 893; 230

F I cs .1

i L/e? LL 1 l I L POWER Z6 SUPPLY RESISTANCE SENSOR METHOD OFl\I.\NUFA(TTL'RIi OF AN EXHAUST GAS SENSOR FOR AN AIR-FUEL RATIO SENSINGSYSTEM This is a continuation of application Ser. 198,515. filed Nov.I5. I971, now abandoned.

SUMMARY OI" THE INVENTION The temperature of the exhaust gases leavingthe combustion chambers of reciprocating internal combustion engines isproportional to the amount of combustion taking place within the engineand this relationship has been used in aircraft for indicating theair-fuel ratio of the combustible mixture being supplied to the engine.Subsequent investigations showed that the thermal conductivities ofvarious exhaust gas components could be used to indicate the proportionof such components in the exhaust gases. These investigations producedsystems of the resistance bridge type that compared the thermalconductivity of the exhaust gases with known gas mixtures to indicateeither air-fuel ratio or the combustion efficiency of the engine.

Recent interest in improving the environment by diminishing the quantity.of undesirable components in the exhaust gases of automotove engineshas accentuated investigations into systems for monitoring continuouslythe airfuel ratio of combustible mixtures. These investigations have ledto numerous refinements of the thermal conductivity system. For example.it was found that thermal conductivity varies almost linearly with thecarbon dioxide content of the exhaust gases and carbon dioxide contentin turn is proportional to the airfuel ratio. Subsequently it was foundthat the thermal conductivity of the exhaust gases is a function of boththe carbon dioxide content and the hydrogen content. Other approachesinvolved combining thermal conductivity devices with exhaust gastemperature devices.

This invention provides a system for determining the air-fuel ratio ofthe mixture supplied to a combustion mechanism by detecting directly theoxidationreduction characteristics of the exhaust gases. The systemcomprises a sensing member made of a metal compound containing oxygenatoms and having at least two metal oxidation states of approximatelyequal energies that is located in contact with either the air-fuelmixture supplied to the combustion mechanism or the exhaust gasesleaving the mechanism. Two electrodes spaced apart from each other by atleast a portion of the sensing member are attached to the member and toan electrical or electronic device for sensing the electrical resistanceacross the electrodes. The electrical resistance is proportional to theequilibrium oxygen pressure of the gaseous mixture in contact therewithand resistance measurements can be converted directly into the air-fuelratio of the mixture supplied to the combusoxides such as titaniumdioxide. vanadium oxide. chromium oxide. manganese oxide. iron oxide.nickel oxide. cobalt oxide. and rare earth metal oxides such as cc riumoxide. praseodymium oxide. etc. Oxides of the metals are preferablebecause the ceramic properties thereof provide relatively long usefullives at higher operating temperatures and of the inherent presence ofoxygen atoms. Other compounds and mixtures of the oxides with each otherand with the other compounds also can be used. Energies of the twooxidation states of the metals must be sufficiently close to permitreversal by changes in the equilibrium oxygen pressure of the gases atoperating temperature. Simple empirical tests can be used to determinethe required relationship.

Scnsing members made from the metal compounds preferably are located inthe exhaust gases leaving the combustion mechanism because the exhaustgases approximate more closely the desired operating temperatures of themembers and do not contain any unvapor' ized fuel. The system of theinvention is useful particw larly in measuring and controlling theair-fuel ratio of the combustible mixture being supplied to an internalcombustion engine.

The sensing member preferably is a relatively thin plate made fromsintered particles of the desired metal compound. Such plates typicallyhave a density of about -95 percent of theoretical; plates of lowerdensity operate effectively but tend to be frangible while plates ofhigher density. including theoretical density. exhibit decreasingresponse times. Particle size affects response time only; a wide varietyof particles sizes can be used. The electrodes are attached to a surfaceof the plate or embedded within the plate. One preferred constructioninvolves sandwiching the electrodes between two green ceramic plates andfiring the assembly into a unitary structure.

Maintaining the sensing member within a relatively broad temperaturerange. typically about 600-900C.. produces adequate indications of theair-fuel ratio supplied to an engine despite the fact that temperaturevar iations change the resistance between the electrodes. Temperaturesbelow 600C tend to coat the member with soot and other particulateimpurities while tem peratures above 900C tend to decrease overall life.Accuracy improvements are achieved by associating a controlledelectrical heater with the sensing member to maintain its temperaturewithin a narrower range. A highly useful structure involves a sandwichmade of three green ceramic plates with the electrodes between an outerplate and the middle plate and an electrical resistance wire between themiddle plate and the other outer plate. A thermocouple for temperaturecontrol can be embedded with either the electrodes or the resistancewire.

It is believed that the metal ions of the metal compounds are reduced oroxidized from one oxidation state to the other in proportion to thereducing or oxidizing nature of the exhaust gases. In the case oftitanium dioxide molecules. for example. reduction frees an electronthat conducts current much more readily and thereby reduces theresistance of the portion of the ceramic material located between theelectrical leads.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic illustration ofa reciprocating internal combustion engine showing the installation inthe exhaust pipe of a sensing member ofthis invention.

PK]. 2 is a schematic plan view of a disc-shaped sens ing member of thisinvention showing the electrical connections thereto.

FIG. 3 is a sectional elevation of the diseshaped sensing member of FIG.2 showing the disposition of the electrodes. an electrical resistanceheating wire. and a thermocouple in a sandwich construction.

DETAlLED DESCRIPTION Referring to FIG. 1. a reciprocating internalcombus tion engine includes an intake manifold [2 for deliv ering anair-fuel mixture to the engine combustion chambers (not shown) and anexhaust manifold H for removing the combustion products from the combustion chambers. A carburetor 16 is attached to the intake manifold and anair cleaner I8 is attached to the air inlet of carburetor l6. Carburetor16 receives fuel from a fuel source (not shown). produces an air-fuelmixture and supplies the airfuel mixture to intake manifold 12.

Exhaust manifold I4 is connected to an exhaust pipe 20. Threaded intothe wall ofexhuast pipe 20 is a plug shaped member 22 comprising adisc'shaped ceramic sensing member 24. Three sets of electrical leads26. 27 and 28 extend from the top of plug-shaped member 22.

Turning to FIGS. 2 and 3, sensing member 22 comprises a sandwich ofthree thin ceramic plates 30, 32 and 34. A length ofelectriealresistance wire 36 is sandwiched between plates and 32. Two electrodes38 and 40 and a thermocouple 42 are sandwiched between plates 32 and 34.Electrodes 38 and 40 are spaced apart a considerable distance shown inFIG. 2. The

entire sandwich is fired into a unitary structure by con vcntionalceramic firing techniques.

Electrical leads 26 connect electrodes 38 and 40 to an electricalresistance sensor 44 as shown in FIG. 2. Leads 27 connect the ends ofresistance wire 36 to an electrical power supply 46 and leads 28 connectther mocouple 42 to a control circuit 48 located between the powersupply and one end of the resistance wire.

Each of plates 30, 32 and 34 consists essentially of titanium dioxide.Each plate has a final thickness of about 0.008 inch and a diameter ofabout 0.25 inch.

The plates are made by a cast tape process that comprises casting atitanium dioxide slurry onto a plastic carrier tape. evaporating thevehicle from the slurry. stripping the plastic tape and punching discsfrom the remaining green ceramic layer. Titanium dioxide powder having aparticle size of 325 mesh is used to make the slurry although subsequentgrinding steps might reduce the final particle size.

Resistance wire 36 typically is made of platinum al loyed with about 13%rhodium and is about 0.008 inches in diameter. Electrodes 38 and 40typically are made of platinum and are about 0.008 inches in diameter.Thermocouple 42 is a gold-palladium-platinum and gold palladiumcombination. The green plates. resistance wire, electrodes andthermocouple are sandwiched together as shown and pressed at about[0,000 psi. After pressing. the assembly is fired at about 23002500F.for 1-2 hours.

The resulting disc is installed in the exhaust conduit of areciprocating internal combustion engine where exhaust gases will heatthe disc to about 700 C. When the engine is supplied with an air-fuelmixture of about 13: l. the resistance across the electrodes is about2000 ohms. (hanging the airt'ucl ratio to l4:l without changing anyother engine parameters increases the rcsistance to about 7000 ohms. Anair tuel ratio of [5:1 produces a resistance of 20.000 ohms. Otherembodiments produce resistances ranging from I00 ohms at alt air fuelratio ot l 1:] to 500.000 ohms at a ratio of 16: I.

Actual values of electrical resistance of course depend also on thedistance between the electrodes and temperature. but these factors shiftthe entire resistance vs. air-fuel ratio curve without affectingsignifi' cantly the shape of the curve. Resistance changes rap idly inthe vicinity of the stoichiometric air-fuel ratios and considerabletemperature variations can be tolerated when measurements are being madein that vicinity.

A wide variety of materials can be used to make the elctrodes.resistance wire and thermocouple used in the sensing member. The sensingmember also can be formed in a wide variety of sizes and shapesincluding cylinders. squares. rectangles. etc.

Thus this invention provides a system for rapidly and accuratelymeasuring the air-fuel ratio of combustible mixtures supplied tocombustion mechanisms. The system is useful not only for analyticalpurposes but also as an element in a control loop for automaticallycontrolling air or fuel supplies to produce or maintain desired airfuelratios.

We claim:

1. A process for preparing an exhaust gas sensing member comprising thesteps of forming a slurry of a major portion of particles of atransition metal oxide;

drying the slurry into a sheet;

forming plates from the dried slurry;

sandwiching spaced electrodes between a pair of said plates. and

tiring the sandwiched plates into a unitary structure.

2. The process of claim 1 including the step of sandwiching anelectrical heating means between a third plate and one of said pair ofplates prior to the step of firing.

3. The process of claim 1 wherein the slurry is formed of particles ofthe transition metal oxide capable of being fired to achieve a densityof from about of theoretical to about 959? of theoretical.

4. The process of claim 3 wherein the slurry is formed of 325 meshparticles of the transition metal oxide.

5. The process of claim 3 wherein the sandwiched plates are fired at atemperature of from about 2300F to about 2S00F for a period of time lessthan about two hours.

6. The process of claim I wherein the step of sandwiching comprises thesteps of: placing a pair of plates in confronting relation on oppositesides of a pair of spaced electrodes; and pressing said plates togetherunder a pressure of about [0,000

psi.

1. A PROCESS FOR PREPARING AN EXHAUST GAS SENSING MEMBER COMPRISING THESTEPS OF FORMING A SLURRY OF A MAJOR PORTION OF PARTICLES OF ATRANSITION METAL OXIDE, DRYING THE SLURRY INTO A SHEET, FORMING PLATESFROM THE DRIED SLURRY, SANDWICHING SPACED ELECTRODES BETWEEN A PAIR OFSAID PLATES, AND FIRING THE SANDWICHED PLATES INTO A UNITARY STRUCTURE.2. The process of claim 1 including the step of sandwiching anelectrical heating means between a third plate and one of said pair ofplates prior to the step of firing.
 3. The process of claim 1 whereinthe slurry is formed of particles of the transition metal oxide capableof being fired to achieve a density of from about 80% of theoretical toabout 95% of theoretical.
 4. The process of claim 3 wherein the slurryis formed of -325 mesh particles of the transition metal oxide.
 5. Theprocess of claim 3 wherein the sandwiched plates are fired at atemperature of from about 2300*F to about 2500*F for a period of timeless than about two hours.
 6. The process of claim 1 wherein the step ofsandwiching comprises the steps of: placing a pair of plates inconfronting relation on opposite sides of a pair of spaced electrodes;and pressing said plates together under a pressure of about 10,000 psi.